Most, if not all, operational amplifiers include a differential input stage responsive to a differential input signal for providing a differential output current which is typically converted through a differential to single-ended converter to a representative output voltage. If the differential input signal is balanced, then the differential output current of the differential input stage is zero. As the differential input signal moves away from a balanced state, the differential output current increases in magnitude accordingly and increases the output voltage. Ideally, the differential input stage produces a non-zero differential output current only as the input signal becomes unbalances; however, conventional differential input stages are susceptible to common mode signals appearing at the inputs thereof which can also produce a non-zero output current and cause an undesirable shift in the output voltage.
The term "common mode rejection" is a measure of how well the differential input stage of the operational amplifier rejects the common mode input signal, appearing as in-phase signals of equal magnitude at both inputs simultaneously and producing an undesirable shift in the output signal of the differential to single-ended converter. The ratio of the common mode input voltage to the output voltage, typically expressed in decibels (dB), indicates the extent to which the operational amplifier maintains zero output voltage given a common mode input signal. A typical common mode rejection ratio may be 100 dB.
Many techniques have been devised for improving the common mode rejection ratio, one of which is covered in the discussion of the prior art as a cascode current source (two transistors in series) for providing a high impedance at the common source terminals of the differential transistors of the input stage. The purpose of the high impedance at the sources of the differential input transistors is to suppress any incremental current flow induced by the common mode input signal which is a principal factor causing the shift in the output signal. The prior art solutions such as the cascode current source continue to have one or more problems with providing satisfactory common mode rejection in that it is difficult to make the impedance of the cascode current source large enough to suppress the incremental current. Moreover, the two serially coupled transistors of the cascode current source limit the dynamic range of the differential input signal. A more desirable approach would be to provide a separate source for the incremental current, rather than attempt to suppress it, such that the incremental output current of the differential input stage does not flow through the differential to single-ended converter and therefore does not effect the output voltage thereof.
Hence, there is a need for a differential input stage having improved common mode rejection for providing substantially zero output signal in the presence of a common mode input signal over the full range of the differential input signal.